A magnetic tunnel junction (MTJ) device is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. One of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed as memory cells for solid state memory and as external magnetic field sensors, such as MR read sensors or heads for magnetic recording systems. The response of the MTJ device is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, a ferromagnetic layer whose magnetic moment is fixed or prevented from rotation, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material comprising the ferromagnetic layer at the interface of the ferromagnetic layer with the tunnel barrier layer. The first ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the second ferromagnetic layer. Usually, when the magnetic moment of the second ferromagnetic layer is parallel to the magnetic moment of the first ferromagnetic layer, there are more available electronic states than when the magnetic moment of the second ferromagnetic layer is aligned antiparallel to that of the first ferromagnetic layer. The tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. Thus, the electrical resistance of the MTJ depends on both the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers.
For a memory cell application one of the ferromagnetic layers in the MTJ has its magnetic moment fixed or pinned so as to be parallel or antiparallel to the magnetic moment of the other free or sensing ferromagnetic layer in the absence of an applied magnetic field within the cell. For a MR field sensor or read head application one of the ferromagnetic layers has its magnetic moment fixed or pinned so as to be generally perpendicular to the magnetic moment of the free or sensing ferromagnetic layer in the absence of an external magnetic field.
A MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used as a MR read head for reading data in magnetic recording disk drives, operates on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy (Ni.sub.81 Fe.sub.19). A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external magnetic is field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element and a corresponding change in the sensed current or voltage.
The use of an MTJ device as a MR read head has been described in U.S. Pat. No. 5,390,061. One of the problems with such a MR read head, however, lies in developing a structure that generates an output signal that is both stable and linear with the magnetic field strength from the recorded medium. If some means is not used to stabilize the sensing ferromagnetic layer of the MTJ, i.e., to maintain it in a single magnetic domain state, the domain walls of magnetic domains will shift positions within the sensing ferromagnetic layer, causing noise which reduces the signal-to-noise ratio. This may give rise to a non-reproducible response of the head, when a linear response is required. Similarly the response of the MTJ MR read head must be approximately symmetric with regard to positive and negative sense fields.
The problem of maintaining a single magnetic domain state is especially difficult in the case of an MTJ MR read head because, unlike an AMR sensor, the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, and thus any metallic materials used to stabilize the sensing ferromagnetic layer that may come in contact with the ferromagnetic layers in the MTJ will short circuit the electrical resistance of the MTJ. IBM's U.S. Pat. No. 5,729,410 describes a MTJ MR read head with ferromagnetic material for longitudinally stabilizing or biasing the sensing ferromagnetic layer, wherein the biasing material is located outside the MTJ stack and separated from the stack by electrically insulating material.
What is needed is an MTJ device that has a stable and linear output and can thus function as an MTJ MR read head that provides a linear response to the magnetic fields from the recorded medium.